Sealing glasses are commonly used in the construction of high temperature electronic devices such as oxygen sensor units which are used to measure the concentration of oxygen remaining in exhaust gases discharged from internal combustion engines and blast furnaces. U.S. Pat. Nos. 4,571,285, 4,596,132, 5,670,032, and 5,739,414 each describe various oxygen sensor units employing sealing glasses. The purpose of sealing glasses in these applications is to isolate the sensing portion of the oxygen sensor unit from an air reference chamber within the sensor unit. Upon firing, the glass seal forming composition melts and flows to join together the materials from which the sensor unit is constructed and form a hermetic seal. The glass seal thus formed prevents exhaust gas in the sensing portion of the sensor unit from leaking into and contaminating the air reference within the sensor unit. Any leakage of exhaust gas into the air reference within the sensor unit would detrimentally affect the validity of the measurement output of the sensor.
It is generally known in the art that in order to produce an effective glass seal between materials, a sealing glass composition must possess a coefficient of thermal expansion which is relatively intermediate between the coefficients of thermal expansion of the materials being sealed together. In general, oxygen sensor units are constructed of ceramic materials such as zirconia, alumina, fosterite and steatite, and metallic materials such as carbon and alloy steels, stainless steel and Inconel.TM.. Joining ceramic materials such as zirconia, alumina, or steatite to metallic materials such as steel or Inconel.TM. requires sealing glasses to possess a relatively wide range of coefficient of thermal expansion, i.e. in the range of about 45.times.10.sup.-7 /.degree. C. to about 95.times.10.sup.-7 /.degree. C., more closely matching the coefficients of thermal expansion of the ceramic material in order to minimize the risk of fracture and residual stress.
The prior art describes several sealing glasses having relatively high coefficients of thermal expansion. Representative examples include the sealing glasses disclosed in U.S. Pat. Nos. 2,642,633; 3,123,470; 3,258,350; and 3,951,669. However, these prior art sealing glasses possess relatively low softening temperatures of about 500.degree. C., making them unsuitable for use in high temperature applications such as in oxygen sensor units where the exhaust gases being sensed may reach temperatures as high as 900.degree. C. or higher, and the glass seals within the unit may be exposed to temperatures as high as about 650.degree. C. At these high operating temperatures, glass seals formed from prior art sealing glasses deform and flow resulting in seal failure.
In addition to having compatible coefficient of thermal expansion and a relatively high glass transition temperature, in order to be suitable for application in high temperature electronic devices such as oxygen sensor units, a sealing glass composition must form a mechanically and chemically durable glass seal which exhibits high electrical resistance at elevated temperatures. These additional requirements necessitate that the seating glass be comprised of a silicate-based glass or glass ceramic composition, but precludes the sealing glass composition from containing any significant concentrations of alkali metal oxides, or admixtures of electrically conductive metal powders as described in U.S. Pat. No. 4,221,604. Additionally, due to manufacturing process constraints, a sealing glass composition must also be capable of melting and flowing to form a glass seal at a firing temperature between 900.degree. and 1,050.degree. C.